The present invention relates to a grease having improved radiation resistance. More particularly, the present invention relates to a radiation-resistant grease which has bentonite (thickening agent) and an alcohol, ketone or ester (gelling aid) incorporated in a base oil which is made of a mixture of a phenoxyphenoxydiphenyl and a mono- and/or dialkyldiphenyl ether.
Grease is employed in various machines at nuclear facilities such as power plants and fuel reprocessing plants, or at irradiation facilities such as cobalt-irradiating equipment and nuclear accelerators, and is subjected to two types of deterioration, one resulting from aging and the other caused by radiation exposure. The deteriorated grease is replaced at the time of periodic inspections and at other suitable occasions but the frequency of grease replacement is desirably held to a minimum in order to improve the degree of capacity utilization of the facilities and to reduce the frequency of exposure to radiation on the part of personnel. A situation therefore exists to require the development of a grease having superior resistance to radiation.
The greases currently employed in machines at nuclear facilities are roughly divided into two types: the silica gel grease that uses a petroleum based lubricant as a base oil, and the soap base grease that incorporates a petroleum based lubricant or a synthetic oil as a base oil. However, these conventional greases are not possessed of adequate resistance to radiation. If the silica gel grease using a petroleum based lubricant as a base oil is employed in machines at a high dose rate irradiation site, the base oil solidifies as a result of radiation-induced polymerization and its lubricating property is lost within a short time to cause premature wear of the machine. On the other hand, the soap base grease which uses a petroleum based lubricant or a synthetic oil as a base oil will soft as a result of exposure to an increased dose of radiation, and the fluidized grease will leak out of the machine. A possible cause of these phenomena is the loss of radiation resistance not only of the base oil and the thickening agent present in the grease but also of the gel structure which is formed as a result of interaction between the base oil and the thickening agent. It would therefore be clear that in order to provide a grease with enhanced radiation resistance, it is important that not only a base oil and a thickening agent having improved radiation resistance shoul be used but also the characteristic gel structure formed as a result of interaction between the base oil and the thickening agent should be stable to radiation.
The present invention has been accomplished in order to develop a highly radiation-resistant grease which is free from the defects of the conventional products.
In Unexamined Published Japanese Patent Application No. 100197/1984, the present inventors disclosed that an oil based on a mixture of a phenoxyphenoxydiphenyl and a mono-or dialkyldiphenyl ether exhibited superior resistance to radiation. The present inventors later found that a base oil which was composed of such oil, a bentonite-based thickening agent and a gelling aid which was chosen from an alcohol, ketone or carbonate ester would provide a grease having sufficiently improved radiation resistance to ensure reliable performance when used as a grease in machines at irradiation sites.